Request body in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when sending a POST request with JSON body?

Consider this NestJS controller method:

  @Post('create')
  create(@Body() data: any) {
    return { received: data.name };
  }

If you send a POST request to /create with JSON body {"name": "Alice"}, what will be the response?

NestJS

import { Controller, Post, Body } from '@nestjs/common';

@Controller()
export class AppController {
  @Post('create')
  create(@Body() data: any) {
    return { received: data.name };
  }
}

A {"received":null}

B {"received":"Alice"}

C {"received":"{\"name\": \"Alice\"}"}

D 500 Internal Server Error

💡 Hint

Think about how NestJS parses JSON request bodies automatically.

The @Body() decorator extracts the parsed JSON body as an object. So data.name accesses the name property, returning "Alice".

📝 Syntax

intermediate

Which option correctly extracts nested properties from request body?

Given this NestJS controller method:

  @Post('user')
  createUser(@Body() body: any) {
    const city = ???;
    return city;
  }

The request body JSON is {"user": {"address": {"city": "Paris"}}}. Which code correctly assigns city to "Paris"?

NestJS

import { Controller, Post, Body } from '@nestjs/common';

@Controller()
export class AppController {
  @Post('user')
  createUser(@Body() body: any) {
    const city = ???;
    return city;
  }
}

A body.city

B body.address.city

C body.user.address.city

D body.user.city

💡 Hint

Follow the JSON structure step by step.

The JSON has user at top level, then address, then city. So body.user.address.city accesses "Paris".

🔧 Debug

advanced

Why does this NestJS controller method fail to parse JSON body?

Look at this controller method:

  @Post('data')
  receiveData(@Body('info') info: string) {
    return info;
  }

A client sends JSON {"user": {"info": "hello"}} but the method returns undefined. Why?

NestJS

import { Controller, Post, Body } from '@nestjs/common';

@Controller()
export class AppController {
  @Post('data')
  receiveData(@Body('info') info: string) {
    return info;
  }
}

A The client did not send the 'info' property at the root level, so @Body('info') returns undefined.

B The @Body('info') decorator extracts the 'info' property, but the client sent a string instead of JSON.

C The client sent JSON with Content-Type other than application/json, so body parsing failed.

D The method expects a string but the body parser returns an object, causing undefined.

💡 Hint

Check the JSON structure and what @Body('info') expects.

The @Body('info') decorator extracts the 'info' property from the root of the JSON body. If the client sends JSON with 'info' nested inside another object, it returns undefined.

❓ state_output

advanced

What is the output of this controller method with partial body?

Given this method:

  @Post('update')
  update(@Body() body: { id: number; name?: string }) {
    return { id: body.id, name: body.name ?? 'Unknown' };
  }

If the client sends {"id": 5} (no name), what is the response?

NestJS

import { Controller, Post, Body } from '@nestjs/common';

@Controller()
export class AppController {
  @Post('update')
  update(@Body() body: { id: number; name?: string }) {
    return { id: body.id, name: body.name ?? 'Unknown' };
  }
}

A {"id":5,"name":null}

B 400 Bad Request

C {"id":5}

D {"id":5,"name":"Unknown"}

💡 Hint

Look at the nullish coalescing operator ??.

If body.name is missing or undefined, ?? 'Unknown' sets the name to "Unknown".

🧠 Conceptual

expert

Which statement about NestJS request body parsing is TRUE?

Choose the correct statement about how NestJS handles request bodies.

A NestJS automatically parses JSON bodies only if the Content-Type header is 'application/json'.

B NestJS requires manual parsing of JSON bodies using JSON.parse inside controller methods.

C NestJS ignores the request body if the route method uses @Body() decorator.

D NestJS can parse XML request bodies by default without extra configuration.

💡 Hint

Think about how HTTP headers affect body parsing.

By default, NestJS uses body-parser middleware that parses JSON only when Content-Type is 'application/json'. Other types need extra setup.